Julian Heming of the Met Office in Exeter explains how Hurricane Rita has grown so big and whether these storms are getting more frequent.

Where did Hurricane Rita come from?

The initial stage of formation was just off the north of Puerto Rico last week, where there was a disturbance in the atmosphere resulting in storm cloud formation. This caused a depression which moved west towards the Florida Straits, and built up into a tropical storm and then a hurricane. The conditions in the Gulf of Mexico were just right to speed its development into a Category 5 storm.

What conditions make the biggest hurricanes?

The basic condition for formation is a sea temperature about 27C. At this time of year the seas around the Gulf of Mexico are at least, if not more than that, and at present they are around 31C-32C.

The other key ingredient is wind shear: for a hurricane to keep growing you need there to be relatively light winds as you go up through the atmosphere. High winds can break apart the structure of the hurricane before it develops.

In the Gulf of Mexico the conditions have been very favourable for the past month, which is why it is not surprising that another hurricane has followed so soon after Katrina.

What part does global warming have to play in these hurricanes?

Although this has been an exceptional year in terms of the number of storms, there have so far not been as many as there were in 1995, when we had 19. There have been similarly active seasons dating back decades. In both 1960 and 1961 there were two Category 5 storms in the Atlantic region and in 1933 there were 21 storms.

If you discount Ophelia, which grazed the Carolinas, three hurricanes have so far made landfall over the US this year. In 1886, records show that there were seven.

So there is no evidence that there are more storms when looked at globally, but what we may be seeing is an increase in the peak intensity of the strongest ones.

Two research papers published in the past month have suggested an increase in the number of category 4 and 5 storms. Tropical cyclone activity is highly variable, often as a result of natural changes in the atmosphere and ocean, so although this evidence is a start, we are a long way from proving a connection.

Will Rita calm down before hitting the coast?

Once a storm has worked itself up to Category 5 its strength is controlled by tiny changes in the sea temperature below. As Rita progresses towards the coast it will pass over small fluctuations from warm and cold eddies on the surface, and these will determine whether the wind speeds increase or decrease.

The hurricane also has its own set of complex internal dynamics. As it gets stronger, the eye becomes smaller and smaller until it can collapse in on itself. Then a much wider eye will develop and wind strength will drop significantly, before it begins to grow again. The wind speed on hitting land depends very much on its stage in the cycle of weakness and intensity, which is a matter of chance.

Are there many more hurricanes to come?

The season can last until the end of November although it tends to tail off in mid-October when sea temperatures begin to drop. Since each hurricane begins from very small disturbance in the atmosphere it is very difficult to predict whether one will start to form, but if it does it is perfectly possible that there may be more on the way.

Are we going to need a Category 6 soon?

As it stands the greatest observed maximum wind speed is approximately 180mph. Category 5 doesn't really have an upper limit, so at the moment I don't think there is any need for an extra category.

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